Take some spackle and work it with a wet toothbrush to get the soil-like texture. Then, when it dries, paint it mud-colored with cheap acrylics and drybrush with a lighter color to pop out the details.

You can take some sand, take PVA glue on surface, sprinke sand and when it dry you just paint vallejo brown color. Or you can mix sand, PVA glue and brown color and paint with mixture over areas where you want soil.

You can take some sand, take PVA glue on surface, sprinke sand and when it dry you just paint vallejo brown color. Or you can mix sand, PVA glue and brown color and paint with mixture over areas where you want soil.

You can use cork scrapings. Just take a cork from a good wine bottle (real cork not the cheap plastic substitutes) and grind it using a normal manual cheese grinder. Please note that you have the advantage of drinking the whine...The grinding process may be a bit messy so do that inside a plastic bag.Than use the scrapings as you would use the sand. Cover the base with white glue, sprinkle the scrapings, wait to dry, remove the loose scrapings (inside the plastic bag) and paint with highly diluted brown or green acrylic paint.

Like matgc or Emperor, I always use real sand or soil. I paint the diorama base with a mixture of PVA glue, cheap acrylic color and water (about 1:1:1) of the color that matches the sand/soil, and then I just sprinkle the sand/soil on this wet coating.

Another technique is to use spackle, then make tiny impressions in the surface with a wet tooth brush. Paint and highlight with your soil color and a lighter color or two. Then you can use pva glue to make little clumps of sand here and there to render the surface of the base less uniform.